A very suitable process and apparatus useful for purifying and cooling the crude product gases from oil and coal gasification processes is described in U.S. Pat. 4,149,859.
If has been found, however, that in the coal gasification the crude product gases contain many fine ash particles, known as fly ash, of which a considerable quantity is so fine that it cannot be separated off by the cyclone or cyclones. Said particles are then removed from the gas in the water scrubber, and by the recycling of the aqueous suspension formed to the hot gas, said fines are returned into the system. The result is that fines increasingly accumulate in the loop formed by the cyclone(s), water scrubber, recycle loop of the suspension and possibly the heat exchanger(s), if no suspension of fine ash particles is discharged from the purification system. The heat exchanger(s) is/are in said loop if the suspension is not injected into the gas after the heat exchanger(s). Injection of the aqueous suspension into the hot gas before the heat exchanger(s) is advantageous, since the suspension then, in fact, evaporates more readily and the gas is slightly precooled. However, said injection of the suspension before, between or into the heat exchanger(s) is then found to have an extra disadvantage, i.e., the heat exchanger(s) foul as a result of local deposition of fines. Said deposition causes a lower degree of heat transfer because the outlet temperature of the gas from the heat exchanger(s) becomes too high, with the result that the cyclone is damaged, and the occasional coming loose of a deposited layer further causes a sudden change in temperature for the heat exchange tubes with all undesirable possible consequences thereof.
Investigations have shown that just the smallest particles are deposited in the heat exchanger(s), especially in dead corners. This is partly explained from local decreases in velocity of the rapidly flowing gas and partly from electrostatic causes. These explanations, however, have not yet resulted in a solution to the problem.